In a variety of applications of practical importance, filters comprising thin films of a piezoelectric material constitute an attractive alternative to other conventional miniature filters. This is particularly true, for example, in handset applications for telecommunications where reducing both the size and cost of filters is critical.
Compared to other filter techniques, thin-film piezoelectric filters have the potential for superior performance, lower cost and substantial reduction in size. Until now, however, it has been exceedingly difficult to produce the requisite thin films of piezoelectric material in a reproducible low-cost manufacturing process.
To make a high-quality thin-film piezoelectric filter, it is necessary that a film of piezoelectric material be deposited in a prespecified oriented manner. Heretofore, efforts to produce such films have relied on various techniques such as collimation, long source-to-substrate distances (long-throw technique) and controlling the nature and condition of the substrate on which the film is formed. But even such measures have not been successful in consistently producing films having the required piezoelectric characteristics. In particular, collimation and long-throw techniques substantially reduce deposition rates, thereby making the deposition times significantly longer than desired.
Accordingly, efforts have continued by workers skilled in the art directed at trying to devise improved ways of producing oriented thin films of piezoelectric material for filter applications. It was recognized that such efforts, if successful, would improve the quality and lower the cost of thin-film piezoelectric filters.